A variety of technical challenges can arise in the export and import of products, in particular with regard to importing or exporting fluid products to or from shore while providing solutions which are efficient and of reasonable cost. One particular market in strong growth is that of the import and export of liquefied natural gas (LNG). In this market, LNG is carried as cargo on large LNG ships (LNG carriers) and is offloaded from the LNG carrier into pipelines or storage facilities for further use or processing. Traditionally, the LNG is loaded onto or offloaded from the LNG carriers at marine LNG terminals at various shore-side locations.
A typical marine LNG terminal normally has a long jetty (often 1 to 5 km long) arranged a distance away from shore and a mooring arrangement for large LNG ships (e.g. 300 m long) together with associated loading equipment such as LNG loading arms. The jetty is typically provided in sheltered water for instance protected by a breakwater. A pipeline on the jetty may then typically be connected to the cargo manifold of the cargo vessel (e.g. an LNG carrier), using the equipment at the terminal. This arrangement provides calm conditions suitable for offloading or loading LNG. However, the breakwater is a large structure (typically 1 km long or more), and the total costs for a providing a marine LNG terminal of this type may be very significant.
Traditional import terminals for LNG also typically include a vapor return line between onshore or offshore storage tanks and the visiting LNG carrier. The reason for the vapor return line is that the LNG tanks on board the visiting LNG carrier need to be filled with gas when the LNG is unloaded. If the onshore/offshore storage tank at the terminal is located far away from where the LNG carrier is moored, a long vapor return line is needed, and the cost for construction of the vapor return line can be high.
The high costs associated with constructing marine LNG terminals has been a significant challenge for the industry over the last 30-40 years, and so there is a need for improved solutions.
Techniques for offloading or loading fluids at locations at sea and remote from shore have been suggested. These can be beneficial in that cargo vessels may not need to travel to shore to offload the fluid. These techniques can have challenges in how to load or offload the cargo reliably, safely and efficiently, and without excessive expenditure or capital requirements. In particular, they need to cope with demands of inclement weather and high-sea states. Conventional moorings can be susceptible to significant rolling motions if wave, wind and currents or swell approach beam-on to the vessel. LNG carriers can be particularly affected by rolling as the LNG containment system (LNG tanks) may be damaged due to LNG sloshing inside the tanks.
In some proposed solutions, LNG may be loaded onto an LNG carrier from an offshore storage facility such as a large spread-moored floating LNG storage unit. The LNG industry has been searching for decades for a solution to be able to safely offload LNG from a spread-moored unit to a conventional LNG carrier, but without much success. As a result, floating LNG storage units which are typically constructed or may be under planning are generally based on using generally costly swivel and turret systems to provide a rotational mooring which allows the unit to weather-vane in order to obtain a more favorable orientation with respect to the prevailing weather direction.
Due to the weather demands it may not be easy to safely connect the necessary tubing to the LNG carrier for loading or offloading LNG. Relative movements between the carrier and the terminal can make it difficult to position the tubing correctly and safely, and make a connection at the cargo manifold of the LNG carrier. The necessary tubing to be connected to the cargo manifold can be very heavy and cumbersome to handle, particularly under dynamic loads as may result from motions of the sea. Thus, there can be a risk of substantial periods of operational downtime.
In addition to the challenges related to lifting and connection of tubing, provisions for handling emergency situations need to be in place to satisfy requirements for LNG terminals. In present solutions, convention has been for the LNG carrier to disconnect and move away from the terminal if needed in the event of an emergency such as a fire or the like.
An example prior art solution is described in the patent publication WO2015/107147 (Connect LNG). This describes a transfer structure which connects onto a side of an LNG carrier at sea by an attachment system which provides for multiple degrees of freedom of movement between the vessel and the transfer structure. The attachment system is described to operate by way of an attractional force created through electromagnets or suction with additional adaptations to allow the freedom of movement sought with respect to the vessel. While this prior art solution might help in certain respects to provide a pipe for transfer of LNG from the vessel to a storage facility, its applicability may be restricted to particular mooring and offloading contexts and may not always be a feasible, efficient, or cost attractive solution. Transit of the transfer structure to the LNG carrier is described to take place through tug or propeller operation.
In light of the above, the present inventors have identified needs for improvement, particularly for improved systems for cargo vessels for offloading or loading fluid cargo with greater operational uptime, safety, simplicity, and/or efficiency. An aim of the invention is to obviate or at least mitigate drawbacks or difficulties experienced in the prior art.